The overall objective of the work proposed here is to understand how neurotransmitters modulate cellular and physiological processes by interacting with specific cell surface receptors. The focus will be on serotonin [5-hydroxytryptamine, 5HT], a biogenic amine that is involved in a wide array of physiological responses in the central and peripheral nervous system. Serotonin exerts its physiological effects by binding to a family of structurally- and functionally-related cell surface receptors, each having distinct pharmacological properties. In addition, these subtypes couple to different intracellular second messenger signaling pathways. In the brain, serotonin receptors are believed to play a key role in modulating affective and perceptual states, and are sites of action of numerous psychotropic drugs, including LSD. In the spinal cord, serotonin is involved in the central regulation of pain, while in the periphery serotonin modulates enteric reflexes and the contraction of smooth muscle. As such, these receptors are potential targets for the pharmaceutical treatment of affective disorders (obsessive-compulsive behavior, depression and schizophrenia), migraine headaches and pain. Genes encoding three 5HT receptor subtypes (5HT1a, 5HT1c and 5HT2) have now been cloned, permitting a molecular analysis of receptor structure and function. When expressed in the unnatural environment of a fibroblasts, the 5HT1c and 5HT2 receptor subtypes bind ligands and activate intracellular second messenger signaling systems. The first aim of this proposal is to extend the molecular characterization of 5HT receptor subtypes by isolating other members of this gene family using standard recombinant DNA methodologies. The second objective is to use the fibroblast expression system as a means for identifying mutations in these receptors that alter their ligand binding or signal transduction properties. The third aim is to biochemically characterize the intracellular second messenger signaling pathways to which these receptors couple in fibroblasts and neuroblastoma cells in culture. Using these simple in vitro systems as models, it should be possible to further our understanding of how these receptors operate in neurons.